Yes, the OED is great, but is not public domain, and is not even commercially available as an epub, and certainly not in Kobo dictionary format.

The OED's that ship with OSX are so nicely formatted, I wish I could figure out a way to decompile-them, but I can only find tools to go the other way (from stardict etc. to Apple's Dictionary 2.0 format)

You mean only real book owners and those with Wikipedia have access to the lastest OED etc.... i can buy all kinds of ebooks but not e-dictionaries... i guess the ones used on kobo are for young readers...i will not read above Stephen King and 50 Shades of Yellow.....

Still, I wonder how you managed to find out that the .html files in the archive were actually gzip files because gzip (on Linux) as well as 7zip (on Windows) are unable to open the ".html" files from the official Kobo dictionaries.

Sure (wb.html was extracted from the default kobo English dictionary):

Still, I wonder how you managed to find out that the .html files in the archive were actually gzip files because gzip (on Linux) as well as 7zip (on Windows) are unable to open the ".html" files from the official Kobo dictionaries.
Have you found a way to open the official files ? Or are you aware of any way to do it ?

It is a little late, but I did not see the question earlier. Not all kobo dictionaries consist of gzipped html files. Some of them have a different (to me unknown) format.

I think the other format is some kind of private encryption. Those file don't seem to have a common header (as far as I remember). Therefore, linux should not know what they are.
(But of course linux recognizes the gzipped files.)

Still, I wonder how you managed to find out that the .html files in the archive were actually gzip files because gzip (on Linux) as well as 7zip (on Windows) are unable to open the ".html" files from the official Kobo dictionaries.
Have you found a way to open the official files ? Or are you aware of any way to do it ?